U.S. Pat. No. 6,590,710 corresponding to JP-A-2001-228326 discloses an infrared gas sensing apparatus for measuring the concentration of a target gas that absorbs a specific wavelength of infrared light. The gas sensing apparatus includes an infrared source that emits the infrared light, a wavelength tunable filter (i.e., Fabry-Perot filter) that selects the specific wavelength of the infrared light, and an infrared detector that detects the filtered infrared light. The gas sensing apparatus measures the concentration of the target gas based on the amount of the infrared light detected by the infrared detector.
As shown in FIG. 6, the wavelength tunable filter includes a first mirror 3 and a second mirror 4. The first mirror 3 is formed on a silicon substrate 1 through a first oxide film 2. The second mirror 4 is formed on a second oxide film 5, which is formed on the first mirror 3. The first and second mirrors 3, 4 face each other.
A gap H is formed between the first and second mirrors 3, 4 by etching the second oxide film 5 via an etching hole 6. Therefore, the second mirror 4 is displaceable with respect to the first mirror 3 by application of an external force. The gap distance of the gap H is equal to the thickness of the second oxide film 5.
The first and second mirrors 3, 4 are made of polysilicon, for example. The first mirror 3 has a first electrode 7 on one surface. Also, the second mirror 4 has a second electrode 8 on one surface. The first and second electrodes 7, 8 are formed by a highly-concentrated impurity doping applied to the surfaces of the first and second mirrors 3, 4, respectively.
A first external electrode 9 is formed on the first electrode 7 and electrically coupled to the first electrode 7. Also, a second external electrode 10 is formed on the second electrode 8 and electrically coupled to the second electrode 8.
The wavelength tunable filter has a center wavelength λ determined by the gap distance of the gap H, i.e., the thickness of the second oxide film 5. For example, the center wavelength λ is 3100 nanometers (nm). Since the first mirror 3 serves as a lower mirror of the wavelength tunable filter, the optical thickness needs to be equal to a quarter of the center wavelength λ. For example, the second oxide film 5 has the thickness of 592 nm and the refractive index of 1.309. Each of the first and second mirrors 3, 4 has the thickness of 248 nm and the refractive index of 3.125.
When a voltage is applied between the first and second electrodes 7, 8 through the first and external electrodes 9, 10, electrostatic attraction force is produced between the first and second electrodes 7, 8. The second electrode 8 is displaced with respect to the first electrode 7 by the electrostatic attraction force. As a result, the gap distance of the gap H is changed. The gap distance is adjusted by adjusting the voltage applied between the first and second electrodes 7, 8. Therefore, the wavelength tunable filter can select the specific wavelength of the infrared light according to the target gas.
In the wavelength tunable filter shown in FIG. 6, the gap distance of the gap H can be adjusted in three levels so that the wavelength tunable filter can select the specific wavelength from three different wavelengths of the infrared light. Thus, the infrared gas sensing apparatus can detect the concentrations of two components of the target gas with one filter. Therefore, the infrared gas sensing apparatus has a small size and is manufactured at low cost.
However, if a foreign matter enters the gap H and is sandwiched between the first and second electrodes 7, 8, the gap distance of the gap H cannot be adjusted. As a result, the infrared gas sensing apparatus incorrectly detects the concentration of the target gas, because the wavelength tunable filter cannot select the specific wavelength where the target gas absorbs the infrared light.